Method for transitioning communications of a mobile subscriber station from a serving base station to a target base station

ABSTRACT

A method for transitioning communications of a mobile subscriber station from a serving base station to a target base station includes receiving from the mobile subscriber station a transition request message having an identifier which identifies the mobile subscriber station, and transmitting a transition request notification message having the identifier to the target base station. The method further includes receiving a transition request notification response message having information relating to capabilities of the target base station, and transmitting a transition response message having the information to the mobile subscriber station.

TECHNICAL FIELD

The present invention relates to a handover control method in abroadband wireless access system. Although the present invention issuitable for a wide scope of applications, it is particularly suitablefor a base station (BS) to transmit data to a mobile subscriber station(MSS) using at least two frequency allocations (FA).

BACKGROUND ART

Generally, in the IEEE802.16e system of which internal standardizationfor broadband wireless access system is in progress, more efficient datatransmission is enabled using a plurality of frequency allocations(FAs). Namely, unlike the conventional method of transmitting datawithin a band by allocating one frequency band to one base station,several frequency bands are allocated to one base station to increasesubscriber accommodation and to transmit data more efficiently.

FIG. 1 is a diagram of inter-frequency handover within one cell andinter-frequency handover between different cells in a broadband wirelessaccess system supporting multiple FAs.

Referring to FIG. 1, in a multiple FA system, if a mobile subscriberstation receives data using a first frequency band FA1 within one cell,it may be attempted to receive data that need a wider bandwidth such asmultimedia data. In this case, if a resource of the first frequency bandFA1 is already occupied by another mobile subscriber station, it isimpossible to offer a multimedia service to the corresponding mobilesubscriber station via the first frequency band FA1. So, the multimediaservice may be enabled in a manner of allocating a second frequency bandFA2 which is still reserved to the corresponding mobile subscriberstation.

For this, an inter-FA handover is needed. The inter-FA handover, asshown in FIG. 1, can be performed in two ways. Namely, a first case isthat a handover 11 is carried out within one cell from the firstfrequency band FA1 to the second frequency band FA2. And, a second caseis that a handover 12 is carried out from a first frequency band FA1 toa fourth frequency band FA4 in a cell using the first frequency bandFA1.

However, in case of using the multiple FA, it is necessary to performthe inter-FA handover within one cell or between different cells. Yet,an inter-FA handover method has not been proposed yet in case of usingthe multiple FA within one cell.

Besides, in case that a mobile subscriber station performs handover froma serving base station to a handover target base station, it has notbeen decided yet that the handover is performed in which way of themultiple FA provided by the handover target base station. And, how toperform the handover in such a case has not been proposed yet.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention is directed to a handover controlmethod in a broadband wireless access system that substantially obviatesone or more of the problems due to limitations and disadvantages of therelated art.

An object of the present invention is to provide a handover controlmethod in a broadband wireless access system, in which a base stationtransmits data to a mobile subscriber station using at least twofrequency allocations.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims thereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, in abroadband wireless access system supporting multiple frequencyallocations, in which a serving base station controls a handover, ahandover control method according to the present invention includes thesteps of receiving from a mobile subscriber station a handover requestmessage including an identifier of the mobile subscriber station,transmitting a handover request notification message including theidentifier of the mobile subscriber station to a handover target basestation, receiving a handover request notification response messageincluding information for frequency allocation supportable by the targetbase station, and transmitting a handover response message including theinformation for the frequency allocation to the mobile subscriberstation.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, in a broadband wireless access system,in which one base station supports multiple frequency allocation andcontrols a handover between frequency allocations, a handover controlmethod includes the steps of transmitting to a mobile subscriber stationa handover request message requesting that the handover shall beexecuted with a specific frequency allocation provided by the basestation, receiving a handover execution indication message includinginformation for a handover action mode from the mobile subscriberstation, and performing a network reentry procedure with the frequencyallocation based on the action mode.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, in a broadband wireless access systemhaving a base station supporting multiple frequency allocations, inwhich a serving base station controls a handover, a handover controlmethod includes the steps of transmitting to a handover target basestation a handover request notification message including an identifierof a mobile subscriber station which will perform the handover,receiving from the handover target base station a handover notificationresponse message including information for a frequency allocationsupportable for the mobile subscriber station, transmitting to thehandover target base station a handover confirmation message notifyingthat the mobile subscriber station will perform the handover with thesupportable frequency allocation, and transmitting to the mobilesubscriber station a handover request message requesting the handoverwith the frequency allocation of the handover target base station.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is a diagram of inter-frequency handover within one cell andinter-frequency handover between different cells in a broadband wirelessaccess system supporting multiple FAs;

FIG. 2 is a flowchart of a handover process between frequencyallocations (FAs) in different cells according to one embodiment of thepresent invention, in which the handover process is requested by amobile subscriber station;

FIG. 3 is a flowchart of a handover process between frequencyallocations (FAs) within one cell according to one embodiment of thepresent invention, in which the handover process is requested by a basestation;

FIG. 4 is a flowchart of a handover process between frequencyallocations (FAs) in different cells according to one embodiment of thepresent invention, in which the handover process is requested by a basestation; and

FIG. 5 is a flowchart of a handover process between frequencyallocations (FAs) in different cells according to one embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIG. 2 is a flowchart of a handover process between frequencyallocations (FAs) to a different cell according to one embodiment of thepresent invention, in which the handover process is requested by amobile subscriber station.

Referring to FIG. 2, a serving base station supports a first FA FA1 anda second FA FA2 and a target base station supports a third FA FA3 and afourth FA FA4, for example.

A base station informs the information of neighbor base stations to allmobile subscriber stations within a cell via a network topologyadvertisement message (MOB_NBR-ADV) for network configuration associatedinformation (S201). Table 1 shows an example of a network topologyadvertisement message applicable to the present invention. TABLE 1Syntax Size Notes MOB-NBR- ADV_Message_Format( ){ Management MessageType = 8 bits 49 Operator ID 24 bits  Unique ID assigned to the operatorN_NEIGHBORS 8 bits For(j=0; j<N_NEIGHBORS; j++){ Neighbor BS-ID 48 bits FA_ID 8 bits Configuration Change 8 bits Increment each time the Countinformation for the associated neighbor has changed Hysteresis threshold8 bits MAHO report period 8 bits TVL Encoded Neighbor variable TVLspecific information  } }

In Table 1, a field FA_ID indicates what kind of FA is supported by aspecific neighbor base station.

A mobile subscriber station carries out scanning on neighbor basestations for a scanning section using the network topology advertisementmessage to check qualities of channels transmitted from the neighborbase stations (S202).

In doing so, the mobile subscriber station can perform a scanningprocess on all FAs supported by one target base station or can perform ascanning process on one of the entire FAs supported by one target basestation. In the following description, a method of scanning one FA onlyfor one target base station supporting multiple FAs is explained.

First of al, once the scanning process is completed, the mobilesubscriber station selects a handover target base station suitable forhandover from the neighbor base stations. The mobile subscriber stationthen transmits a handover request message (MOB-MSSHO-REQ) includinginformation of the selected target base station to a serving basestation (S203). The serving base station notifies that there exists themobile subscriber station attempting to make a handover to the handovertarget base station via a handover request notification message(HO-pre-notification) (S204). And, the handover request notificationmessage includes a mobile subscriber station identifier (MSSIdentifier), connection parameters, capabilities, information for arequired bandwidth and a quality of service (QoS) and the like.

The handover target base station decides which one of FAs supported bythe handover target base station itself is supportable using theinformation included in the handover request notification message (S205)and then transmits a handover request notification response message(HO-pre-notification-response) including the corresponding result to theserving base station (S206). The handover notification response messageincludes reception confirmation information for the handover requestnotification message, information indicating that the same QoS of theQoS class transmitted via the handover request notification message isoffered, and the like.

In the present embodiment, although the mobile subscriber stationcarries out scanning on the third FA (FA3) among the FAs supported bythe target base station, a case that the FA supportable by the targetbase station is the fourth FA (FA4) is explained as follows.

First of all, in case that the target base station can support thefourth FA (FA4), the handover request notification response messageindicates that the fourth FA (FA4) is supportable. The supportable FAinformation can directly indicate center frequency information or can besimplified using an FA identifier (FA_ID) agreed with each FA.

Table 2 shows an example of a handover request notification responsemessage (HO-pre-notification-response) applicable to the presentinvention. TABLE 2 Field Size Notes Global Header 152 bits  For(j=0;j<Num Records; j++){ MSS unique 48 bits  48-bit unique identifier usedby MSS identifier (as provided by the MSS or by the I- am-host-ofmessage) BW Estimated 8 bits Bandwidth which is provided by BS (toguarantee minimum packet data transmission) TBD how to set this fieldQoS Estimated 8 bits Quality o Service level Unsolicited Grant Service(UGS) Real-time Polling Service (rtPS) Non-real-time Polling Service(nrtPS) Best Effort FA_ID 8 bits The identifier of the FA which isrecommended } Security field TBD A means to authenticate this message

The serving base station having received the handover requestnotification response message from the target base station notifies thetarget base station that the handover shall be executed with the fourthFA (FA4) of the target base station via a handover confirmation message(HO-confirm) (S207). And, the serving base station, transmits a handoverresponse message (MOB-HO-RSP) including the center frequency of thefourth FA (FA4) or the previously agreed FA_ID to the mobile subscriberstation (S208). Table 3 shows an example of a handover response messageapplicable to the present invention. TABLE 3 Syntax Size Notes MOB-NBR-ADV_Message_Format( ){ Management Message 8 Type = 54 bits Estimated HOstart 8 bits For(j=0; Neighbor base station shall j<N_Recommended; j++){be presented in an order such that the first presented is the one mostrecommended and the last presented is the least recommended.N_Recommended can be derived from the known length of the message.Neighbor BS-ID 48 bits FA_ID 8 The Identifier of the FA bits which isrecommended Service level 8 predication bits  } }

The mobile subscriber station having received the handover responsemessage transmits a handover execution indication message that indicatesa handover execution to the serving base station (S209). The servingbase station having received the message then releases a call for themobile subscriber station (S210).

Table 4 is an example of a handover execution indication messageapplicable to the present invention. TABLE 4 Syntax Size Notes MOB-BSHO-RSP_Message_Format( ){ Management Message Type =  8 bits 54 reserved  6bits Reserved; shall be set to zero HO_IND_type  2 bits 00: Serving BSrelease 02: HO cancel 10: HO reject 11: reserved BS_ID 48 bits ServingBS ID or Target BS ID FA_ID  8 bits The Identifier of the FA HMAC Tuple21 bytes }

In Table 4, in case that the mobile subscriber station performs handoverwith another FA within the same cell, a field BS_ID indicates an ID ofthe serving base station and a field FA_ID indicates an identifier ofthe FA to which the handover will be made.

The mobile subscriber station receives fast ranging IE (Past Ranging_IE)from the selected fourth FA (FA4) of the handover target base station(S211) and requests a ranging to the target base station (S212) usingthe received IE (S212). The target base station then responds to theranging (S213) so that the handover is performed.

FIG. 3 is a flowchart of a handover process between frequencyallocations (FAs) within one cell according to one embodiment of thepresent invention, in which the handover process is requested by a basestation.

Referring to FIG. 3, a base station can arbitrarily request a handoverwithout a handover request from a mobile subscriber station (UnsolicitedHandover). Namely, the base station transmits a handover request message(MOB-BSHO-REQ) to the mobile subscriber station (S301) so that themobile subscriber station can perform handover via another FA within thebase station. Yet, the mobile subscriber station having received thehandover request message does not have to perform the handover all thetime but can reject the handover requested from the base station.

The handover request message transmitted from the base station to themobile subscriber station includes information indicating whether thehandover takes place between FAs within one cell or between FAs with therelation to another cell.

Moreover, information for an action mode is included in the handoverrequest message. And, the action mode can include five cases as follows,

-   -   (1) Handover supported by backbone data exchange on network    -   (2) Handover not by backbone data exchange on network    -   (3) A case that directly takes place with a different FA within        a cell of a serving base station    -   (4) A case of undergoing a network entry process for a new FA if        handover takes place with a different FA within a cell of a        serving base station    -   (5) A case of enabling a mobile subscriber station to perform        fast handover in a manner that a serving base station previously        allocates a non-contention ranging area necessary for a handover        of the mobile subscriber station and an FA

Hence, each of the cases (1) and (2) corresponds to a case that thehandover is performed via the FA of the target base station and each ofthe cases (3) to (5) corresponds to a case that the handover isperformed via a different FA within the serving base station.

Table 5 shows an example of a handover request message transmitted froma base station to a mobile subscriber station. TABLE 5 Syntax Size NotesMOB-BSHO- REQ_Message_Format( ){ Management Message 8 Type = 52 bits FID1 0: Another FA in Serving BS bit 1: FA in recommended BS If(FID==0){FA_ID 8 The Identifier of the FA bits }else{ N_Recommended 3 Number ofbase stations shall bits be presented in an order such that the firstpresented is the one most recommended and the last presented is theleast recommended. Neighbor BS-ID 48 bits FA_ID 8 The Identifier of theFA bits service level 8 prediction bits  } } Action mode 4 0x00: networkassistant HO bits support 0x01: non network assistant HO support 0x02:direct use of own another FA_ID in Serving BS 0x03: network entry withanother FA_ID in Serving BS 0x04: non-contention initial- ranging withanother FA_ID in Serving BS 0x05 to 0x0f: reserved }

The handover request message, as shown in Table 5, can include the fivekinds of action modes.

The mobile subscriber station having received the handover requestmessage transmits a handover execution indication message(MOB-HO-Indication) notifying the handover execution to the serving basestation (S302). And, the mobile subscriber station performs a networkreentry procedure with the serving base station (S303). In doing so, themobile subscriber station carries out the network reentry procedure onthe FA2 according to one of the five kinds of action modes.

FIG. 4 is a flowchart of a handover process between frequencyallocations (FAs) in different cells according to one embodiment of thepresent invention, in which the handover process is requested by a basestation.

Referring to FIG. 4, a serving base station notifies that there exists amobile subscriber station attempting to make a handover to a handovertarget base station via a handover request notification message(HO-pre-notification) (S401). And, the handover request notificationmessage includes a mobile subscriber station identifier (MSSIdentifier), connection parameters, capabilities, information for arequired bandwidth and a quality of service (QoS) and the like.

The handover target base station decides which one of FAs supported bythe handover target base station itself is supportable for the mobilesubscriber station using the information included in the handoverrequest notification message (S402) and then transmits a handoverrequest notification response message (HO-pre-notification-response)including the corresponding result to the serving base station (S403).

The handover notification response message includes receptionconfirmation information for the handover request notification message,information indicating that the same QoS of the QoS class transmittedvia the handover request notification message is offered, and the like(S403).

The supportable FA information can directly indicate center frequencyinformation or can be simplified using an FA identifier (FA_ID) agreedwith each FA.

The serving base station having received the response message from thetarget base station notifies the target base station that the mobilesubscriber station will make handover with a specific FA of the targetbase station via a handover confirmation message (S404).

And, the serving base station transmits information for the specific FAto the mobile subscriber station using a handover request message(MOB-BSHO-REQ) (S405). The mobile subscriber station having received thehandover request message then transmits a handover execution indicationmessage (MOB-HO-IND) notifying the handover execution to the servingbase station (S406).

The mobile subscriber station performs scanning on neighbor basestations (S407) and decides whether to make handover with the specificFA of the target base station. In case of performing the handover withthe specific FA, the mobile subscriber station receives fast ranging IEfor the specific FA from the target base station (S408).

FIG. 5 is a flowchart of a handover process between frequencyallocations (FAs) in different cells according to one embodiment of thepresent invention.

Referring to FIG. 5, a serving base station transmits are-registration/release command message (DREG-CMD) to a mobilesubscriber station (S501). The re-registration/release command message(DREG-CMD) includes action codes having information for action contentsperformed by the mobile subscriber station in performing handover. Table6 shows an example of action codes and action contents following theaction codes, respectively, TABLE 6 Action Code Action 0x01 SS shallleave the current channel and attempt to access another channel 0x01 SSshall listen to the current channel but shall not transmit until anRES-CMD message or DREG-CMD with Action Code 0x00 is received. 0x02 SSshall listen to the current channel but only transmit on the Basic,Primary Management and Secondary Management connections. 0x03 SS shallreturn to normal operation and may transmit any of its activeconnections. 0x04 SS shall terminate current Normal Operation with theBS; the BS shall transmit this action code only in response to any SSDREG- REQ 0x05 Required MSS deregistration from serving BS and requestinitiation of MSS Idle Mode 0x06 The MSS may retransmit the DREG-REQmessage after the time duration (REQ-duration) given by 0x07 The MSSshall not retransmit the DREG-REQ message and shall await the DREG-CMDmessage 0x08 The MSS shall leave the current FA and move another FAwithout network reentry. 0x09 The MSS shall leave the current FA andattempt to non-contention based access another FA. 0x0a The MSS shallleave the current FA and attempt to access another FA.

In case that the action code (Action Code) included in there-registration/release command message is 0x08, the mobile subscriberstation releases FA1 from the serving base station (S503) and performs aconnection to FA2 in a handover target base station (S504). In doing so,since there exists no network reentry procedure, a MAC address and CID(Connection Identifier) of the mobile subscriber station are maintained.

In case that the action code (Action Code) included in there-registration/release command message is 0x09, the mobile subscriberstation releases FA3 from the serving base station (S505) and performs aconnection to FA2 in the handover target base station. In doing so, themobile subscriber station receives fast ranging IE (S506). Namely, thetarget base station performs a connection to the mobile subscriberstation without contention in a manner of previously allocating FA2 tothe mobile subscriber station that performs the handover. Hence, the MACaddress of the mobile subscriber station is maintained as it is.

In case that the action code (Action Code) included in there-registration/release command message is 0x0a, the mobile subscriberstation releases FA1 from the serving base station (S508) and newlyperforms the network reentry procedure on FA2 in the target base station(S509). Hence, the previous information such as the MAC address and CIDof the mobile subscriber station are discarded.

INDUSTRIAL APPLICABILITY

Accordingly, in the broadband wireless access system supporting themultiple FA, the present invention provides the following effects oradvantages.

First of all, the present invention enables the handover between FAswithin one cell.

Secondly, the present invention enables the handover between FAs indifferent cells.

While the present invention has been described and illustrated hereinwith reference to the preferred embodiments thereof, it will be apparentto those skilled in the art that various modifications and variationscan be made therein without departing from the spirit and scope of theinvention. Thus, it is intended that the present invention covers themodifications and variations of this invention that come within thescope of the appended claims and their equivalents.

1-21. (canceled)
 22. A method for transitioning communications of amobile subscriber station from a serving base station to a target basestation, the method comprising: receiving from the mobile subscriberstation a transition request message comprising an identifier whichidentifies the mobile subscriber station; transmitting a transitionrequest notification message comprising the identifier to the targetbase station; receiving a transition request notification responsemessage comprising information relating to capabilities of the targetbase station; and transmitting a transition response message comprisingthe information to the mobile subscriber station.
 23. The methodaccording to claim 22, wherein the information comprises frequencyallocation supportable by the target base station.
 24. The methodaccording to claim 22, wherein the information comprises resourceallocation supportable by the target base station.
 25. The methodaccording to claim 22, wherein the transitioning communications of themobile subscriber station comprises a handoff of the mobile subscriberstation from the serving base station to the target base station. 26.The method according to claim 22, wherein the serving base station is acurrent base station with which the mobile subscriber stationcommunicates prior to the transitioning.
 27. The method according toclaim 22, wherein the identifier comprises a unique identifier used bythe mobile subscriber station.
 28. The method according to claim 22,further comprising: transmitting the transition request notificationmessage from the serving base station to the target base station. 29.The method according to claim 22, further comprising: transmitting thetransition response message from the serving base station to the mobilesubscriber station.
 30. The method according to claim 22, furthercomprising: generating the transition request notification responsemessage responsive to the receiving of the transition requestnotification message by the target base station.
 31. A method fortransitioning communications of a mobile subscriber station from aserving base station to a target base station, the method comprising:communicating a transition request to the target base station via theserving base station, the transition request comprising an identifierwhich identifies the mobile subscriber station; and receiving atransition response message generated responsive to the transitionrequest and comprising information relating to capabilities of thetarget base station.
 32. The method according to claim 31, wherein theinformation comprises frequency allocation supportable by the targetbase station.
 33. The method according to claim 31, wherein theinformation comprises resource allocation supportable by the target basestation.
 34. The method according to claim 31, wherein the transitioningcommunications of the mobile subscriber station comprises a handoff ofthe mobile subscriber station from the serving base station to thetarget base station.
 35. The method according to claim 31, wherein theserving base station is a current base station with which the mobilesubscriber station communicates prior to the transitioning.
 36. Themethod according to claim 31, wherein the identifier comprises a uniqueidentifier used by the mobile subscriber station.
 37. The methodaccording to claim 31, further comprising: receiving the transitionresponse message from the serving base station.